nmsg_input: export statistics from seqsrc tracking

[Robert Edmonds]

there is no public interface exporting the results from seqsrc tracking. possibly we could have the nmsg_input implementation keep aggregate counters and export them via some new functions such as:

/**
* For UDP datagram socket nmsg_input_t objects, retrieve the total
* number of NMSG containers that have been received.
*
* \param[in] input UDP socket based NMSG input object.
* \param[out] count Total number of NMSG containers received by the
* nmsg_input_t object during its lifetime.
*
* \return #nmsg_res_success
* \return #nmsg_res_failure
*/
nmsg_res
nmsg_input_get_count_container_received(nmsg_input_t input,
                                        uint64_t *count);

/**
* For UDP datagram socket nmsg_input_t objects, retrieve the total
* number of NMSG containers that been dropped. Sequence number
* tracking must have been previously enabled by a call to
* #nmsg_input_set_verify_seqsrc().
*
* \param[in] input UDP socket based NMSG input object.
* \param[out] count Number of NMSG containers determined to have
* been dropped by the nmsg_input_t object since sequence number
* tracking was enabled.
*
* \return #nmsg_res_success
* \return #nmsg_res_failure
*/
nmsg_res
nmsg_input_get_count_container_dropped(nmsg_input_t input,
                                       uint64_t *count);